Abstract

Tectonites from ~2.7-Ga-old Platberg (mid-Ventersdorp) rift-related fault zones in H.J. Joel and Harmony gold mines (southern Free State Goldfield) were dated by the 40Ar-39Ar step-heating technique. A mylonite from a bedding-parallel thrust zone at H.J. Joel Gold Mine yielded an age plateau with ages between ~2.08 Ga and ~2.07 Ga (interpreted age 2076 ± 5 Ma). A cataclasite from a north-striking shear zone yielded a partial (60%) age plateau with ~2.095 to ~2.065 Ga ages, resulting in an interpreted age of 2088 ± 7 Ma. Both partially recrystallised tectonites exhibited 39Ar-loss at, or more recent than, ~1.83 to ~1.75 Ga. A completely recrystallised ultracataclasite from a major northeast-striking fault zone yielded only ages <~1.75 Ga. Seemingly two phases of low-temperature resetting at ~1.83 to ~1.75 Ga and ~1.35 to ~1.20 Ga affected this sample.

The argon age systematics of mylonitic fault rocks from Harmony Gold Mine that are crosscut by younger ultracataclastic-pseudotachylitic breccias and that are developed along individual shear planes of the Merriespruit Thrust and De Bron Fault zones are disturbed. Only one mylonite yielded a plateau age of 2019 ± 8 Ma, which is within errors of the age of breccia development due to the Vredefort impact event. Most of these mylonites record ~1.83 to ~1.75 Ga ages. Several other ultracataclastic-pseudotachylitic breccias could not be precisely dated, as the samples do not contain sufficient potassium. The spectra of these samples are saddle-shaped with strong indications of significant excess argon. Saddle ages of 2841 ± 15 Ma and 2020 ± 20 Ma were obtained for one sample each from the De Bron and Merriespruit fault zones.

Ages in the interval ~2.095 to ~2.065 Ga document a thermal overprinting event of pre-Bushveld-age, which led to complete resetting of the isotope system of older cataclasites and mylonites. This thermo-metamorphic event is believed to represent the initial phase of the Palaeoproterozoic Magondi (Ubendian) Orogeny at ~2.1 to 2.06 Ga along the western margin of the Zimbabwe Craton, which resulted in intense magmatic activity throughout the northern and central Kaapvaal Craton. Pre- to early orogenic magmatism in the Kaapvaal Craton culminated in the structurally controlled emplacement of the Bushveld Igneous Complex, which caused regionally persistent greenschist facies metamorphism (~2 to 3 kbar; 350°C) and the formation of peak-metamorphic mineral assemblages in West and Central Rand Group lithologies in the northern and central Witwatersrand Basin at ~2.06 to ~2.05 Ga, but appears not to have thermo-metamorphically effected the southern portion of the basin.

Besides geological evidence for Vredefort-related generation of ultracataclastic-pseudotachylitic breccias along some of the major fault zones in the Free State Goldfield, ages that could be related to the Vredefort impact event at ~2.02 Ga were obtained from ultracataclastic-pseudotachylitic breccias cutting across earlier developed cataclasites and mylonites along the De Bron Fault and Merriespruit Thrust Zone. Ages <~2 Ga represent post-Vredefort tectono-thermal or hydrothermal events that have previously also been documented from breccias in the north-central Witwatersrand Basin. Expected Platberg formation ages of ~2.71 Ga were not obtained from any of the analysed cataclastic and mylonitic fault rock specimens, due to the intense and almost complete resetting of their isotopic systems during later (pre-Bushveld, Bushveld, Vredefort, post-Vredefort) tectono-thermal events. These results clearly indicate that post-2 Ga tectono-hydrothermal activity affected the central part of the Kaapvaal Craton. This activity is interpreted as a manifestation of orogenic activity in the Proterozoic mobile belts around the craton margins during the Eburnean (Kheis) and Kibaran/Grenville (Namaqua-Natal) orogenies.

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